The invention relates to a process of manufacture of a composite material, consisting of a matrix component made from one or more metals or alloys out of groups of IVb to VIb of the periodic table, as well as of a strengthening component.
The high-melting metals titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten and rhenium as well as their alloys exhibit high tensile strength and creep strength at elevated temperatures. The upper limits of application of these materials range from about 650xc2x0 C. for advanced titanium alloys to about 2200xc2x0 C. for tungsten alloys. It is characteristic for these materials that these limiting temperatures increase with their density. Especially with regard to components in aerospace, therefore, high-temperature applications of these materials are often ruled out because of their high densities.
Many efforts have been made to improve the hot-strength of the high-melting materials in order to increase their general range of application, and specifically to impart the required hot-strength to those alloys with lower densities preferentially employed for accelerated parts such as in aerospace. Well-known mechanisms accomplishing this are solid-solution and dispersion strengthening as well as precipitation hardening. Full exploitation of these effects requires the strengthening component to be present on an atomic scale (solid solution) or in the sub-micrometer range. For the case of precipitations or dispersoids the limits of these mechanisms are reached whenever the strengthening component either dissolves in the matrix or coalesces to larger particles. Based on one or more of these effects it was possible to push upward by the order of 100xc2x0 to a few 100xc2x0 C. the operating temperatures of high-melting alloys. But even then the high-temperature strength attained by these alloys often proved insufficient with regard to the requirements of demanding high-temperature applications.
Light metal and copper-based alloys are routinely reinforced by additions of filaments, platelets, whiskers etc. Generally the manufacture of such alloys is based on melt-metallurgy, sometimes also on pow der metallurgical techniques. Especially the useage of whiskers brings about considerable health hazards.
Manufacture of comparable composite materials based on a matrix made of refractory metals has so far been very limited. The major reason for this lies in the fact that owing to the high processing temperatures which would be required for refractory metals melt-metallurgical processes can hardly be employed. But even the powder-metallurgical processes in use for refractory metals are often not applicable because of the insufficient stability of the available reinforcements at the high temperatures and the long durations required at the stage of sintering.
There has been reported the addition of platelets of hard materials such as titanium diboride or titanium carbide to titanium alloys, whereby an increase in strength by about 30% could be achieved (xe2x80x9cParticulate-Reinforced Titanium Alloy Composites Economically Formed by Combined Cold and Hot Isostatic Pressingxe2x80x9d, Industrial Heating 1993, by Stanley Abkowitz et al ).
There has further been reported the reinforcement of niobium or niobium alloys by incorporation of high-hot-strength wires made of a tungsten/rhenium/bafnium carbide alloy, with a volume content of the latter of more than 50 vol % (see Titran et al, in xe2x80x9cRefractory Metals: State of the Art 1988xe2x80x9d, ed. The Minerals and Metal Society, 1989). Thereby a very significant improvement in strength was achieved, especially in the range of high temperatures up to 1800xc2x0 C. The disadvantage is that this increase in hot strength is gained at the expense of a strongly increased density of this material. Moreover the wires are not thermodynamically stable and the material ages by way of interdiffusion.
U.S. Pat. No. 3,270,412 describes a process for the manufacture of dispersoid-strengthened metallic materials by multiple rolling of stacks of thin metal foils (e.g. Al or Ti) covered with particles or a thin film of a dispersoid material. Owing to the high deformation encountered by the stack there results a material homogeneously interspersed with particles of the dispersoid ( diameter less than 1 xcexcm ). Hence this patent teaches the manufacture of a dispersoid-strengthened material, and not that of a composite material.
In Pat. CA-999 057 there is disclosed a process for the manufacture of multi-phase alloys by way of coating of or lamination of thin sheets of material A ( =matrix ) with a material B (metal or oxide ) and subsequent heat treatment aiming for the formation of intermetallic phases AxBy within the matrix material A. Here material B, deposited e.g. as a coating, diffuses into the matrix and reacts with the latter, forming new intermetallic phases. A serious disadvantage of this concept if applied to high-temperature applications would be that this reaction would further proceed during application and hence no long-term stability of the material properties could be achieved.
A similar idea was put forward in JP 02 133550A. According to this patent an intermetallic compound AxBy is prepared by way of stacking of thin sheets of material A coated with material B, followed by rolling and heat treatment in order to produce the desired alloy by way of diffusion. Although in this case certain process steps that could also lead to a composite material are empoyed, in essence this patent teaches the production of an intermetallic compound.
It is the aim of the present invention to establish a process for the production of a composite material, consisting of a matrix made of one or more metals or alloys thereof chosen from the group Ti, Zr, Hf, V, Nb, Ta, Mo, W and Re as well as of a reinforcing component which circumvents the afore-mentioned limitations.
According to the present invention the composite is produced by forming the matrix component into foils, thin sheets or wires, by coating these with the reinforcing component to a thickness between 1 xcexcm and 100 xcexcm, and by combining a multitude of these foils, thin sheets and/or wires and compacting them unseparably under the action of suitably selected pressures and/or temperatures.
When applying the process according to the present invention there are obtained materials consisting of a multitude of substructures which are put in parallel with regard to the forces exerted during application, and which after their synthesis still exhibit essential morphological features of the original matrix component (the foil, the wire etc. ). Separating these substructures are the undeformed orxe2x80x94depending on the degree of deformationxe2x80x94co-deformed or fragmented layers of the strengthening component. In the latter case these reinforcing fragments attain the form of filaments, platelets or small rods which show a uniform orientation within the matrix.
According to the invention the process will generally be employed to produce a composite material consisting of one single matrix component and one single reinforcing component. But it may also be conceived that the composite will be made up from one or more matrix components combined with one or more different reinforcing components, which allows interesting combinations of materials to be synthesized.
The reinforcing component may consist of one or more compounds or mixtures thereof taken from the group of oxides, carbides, nitrides or borides of the metals of group IVb to VIb as well as of silicon, aluminium and of the rare-earth metals. Furthermore the reinforcing component may consist of a metal, an alloy or an intermetallic compound, or mixtures thereof, selected from the group of niobium, tantalum, chromium, molybdenum, tungsten or rhenium as well as silicon and aluminium, provided that in the case of refractory metals as reinforcing components the latter will have a higher strength than the matrix.
One advantage of the process according to the present invention lies in the fact that the reinforcing component is deposited as a thin, adherent film using established coating processes. In this way all kinds of reinforcing materials become readily accessible at relatively low costs. Moreover the health hazards which are often associated with the production of composite materials are avoided.
The selection of the reinforcing component will firstly be guided by its tensile strength and its modulus of elasticity. In addition the respective coefficients of thermal expansion of reinforcement and matrix must be taken into consideration. Finally the behaviour of the reinforcing component during deformation must be accounted for by suitably selecting the thickness of the reinforcing layer and by adjusting the conditions of deformation. The volume content of the reinforcement will be selected according to the material combination and the required material properties between a few and 50%.
The thicknesses of the foils, sheets or wires used as starting material for the matrix depend on the one hand on the final dimensions of the composite materialxe2x80x94the invention calls for a multilayer stacking or multi-stranded twistingxe2x80x94and on the other hand on the degree of deformation during compaction, on the thermo-mechanical mismatch between matrix and reinforcing component, as well as ultimately on the production costs for the starting material. In most applications a compromise between technical and economical considerations bringing to bear the advantages of the composite material per the present invention will be found at thicknesses of the foils, wires etc between 50 xcexcm and 200 xcexcm.
For the deposition of the reinforcing component any of the well-known processes of coating or surface modification may be considered. The sole requirement is that the coating thickness or the thickness of the surface-modified zone may be reproduced within the limits of 1 xcexcm and 100 xcexcm, and that a dense and flawless layer is reliably achieved.
In case of non-deformable reinforcements the thickness will in general be selected in the lower range between 1 xcexcm and 10 xcexcm. This is the case for most carbides, nitrides, borides and oxides of the transition metals, of the rare-earths as well as of silicon and aluminium. Ductile reinforcing components such as tungsten, rhenium or alloys thereof or with other high-melting materials may be advantageously applied also in the upper thickness range up to 100 xcexcm. The coating thicknesses are in each case selected such that they do not exceed 10% and 50% of the thickness of the prematerial of the matrix for brittle and deformable reinforcements, respectively.
The process according to the invention is carried out in such a way that the reinforcing component is present already at the point of its deposition and that it is sufficiently stable against reactions with the matrix during subsequent production steps as well as during application of the reinforced part. Here xe2x80x9csufficientxe2x80x9d means that the major part of the reinforcing component preserves its chemical composition and further that the minor reaction products do not adversely affect the strength of the composite material.
Particularly well suited for the deposition of the reinforcing component are PVD processes such as Arc Ion Plating or Magnetron Sputtering, which upon suitable selection of deposition parameters yield dense, fine-grained and very strong films of carbides, nitrides, borides and oxides.
For the composite materials produced according to the present invention it was totally unexpected that at low volume contents of the reinforcing component there was observed a strengthening significantly higher than that calculated from the rule-of-mixtures. At the same time in bending tests there was observed for W and Mo reinforced per the present invention a lowering of the apparent ductile-to-brittle transition temperature by several 100xc2x0 C. to below room temperature. In tensile tests the fracture elongation of the composite materials per the present invention as compared to the un-reinforced matrix is reduced, but over the whole range of operating temperatures an elongation  greater than 3% can be maintained.
A preferred embodiment of the present invention employs Nb or Ta or alloys thereof as matrix material and a carbide, oxide or nitride (or mixtures thereof) of Ti, Zr or Hf as reinforcing component. Such composite materials exhibit a very favorable ratio of high-temperature strength to density and are hence particularly well suited for applications in the aerospace sector.
In another preferred embodiment the invention is applied to Mo or W or alloys thereof as matrix material and a carbide, oxide or nitride (or mixtures thereof) of Ti, Zr or Hf as reinforcing component. Such composite materials exhibit high hot-strengths up to very elevated temperatures and can be used advantageously for high-temperature furnace parts.
A particularly well-suited process for the compaction of the assembly of the individual coated matrix components to form the final composite lies in Hot Isostatic Pressing, which may be followed by mechanical working with a low degree of deformation.
A very cost-effective way of compaction of the individual coated matrix components consist of sole mechanical compaction, e.g. by rolling. In this case as a rule higher degrees of deformation in the range between 50% and 70% will be required.
In order to optimize the morphology, e.g. by the formation of a elongated grains, following the compaction of the individual coated matrix components the composite is advantageously subjected to a suitable heat treatment.